The invention relates to a method for repair of a spine from a posterior approach especially for locating an implant between a lower and upper human vertebra. The invention further relates to an intervertebral implant and especially an implant for fusing together adjoining vertebrae bodies.
Since its earliest history, surgery has been an imperfect compromise between the severity of the disease, the goal to cure, and the damage inflicted by the treatment. Spinal fusion surgery with implants is no different. The spine lies deep within the body. It is adjacent to vital structures that are easily damaged. To reach the spine and the pathology to be treated, many bodily structures are often disrupted or destroyed. Supporting bone of the vertebral column must at times be resected and stabilizing ligaments removed. Vessels providing blood supply are oftentimes cut and nerves can be severed.
For certain patients, a lumbar spinal fusion procedure is chosen for treatment, with the combined goals to decompress the painful nerves, to correct a deformity, to immobilize a painfully moving vertebral segment, and to hold it stable in a desired position while the motion segment fuses or grows into a bridge of bone.
To perform a lumber spinal fusion procedure, the surgeon first decides how to approach the spine, how to reach the pathology and how to build a biomechanical fusion construct, which can be composed of inter vertebral cages with or without pedicle fixation. The prior art often uses a midline posterior approach. This requires the surgeon to dissect the posterior lumbar multifidus muscle at a given level, from the spine's posterior elements and to expose the facet joint and lamina. For access, the surgeon must many times dissect levels above and below the level to be treated to enable the surgeon to decompress bone where needed and to enter the spinal canal in order to insert his cages. Dissection of the multifidus from the posterior elements is also the most commonly used method to give the surgeon access to the pedicles for the insertion of the pedicle screws when these are required. In the weeks following surgery, such dissection leads to atrophy and perhaps in itself, a source of additional pain.
Furthermore, at each segment the medial branch of the dorsal ramus, with its accompanying vascular supply, together called the neurovascular bundle, passes across the lamina to supply specific fascicles of the multifidus at each level. The midline surgical approach for a single fusion level typically disrupts the multifidus from the posterior elements of several vertebrae, two vertebrae of the motion segment to be fused, and at least one level above and one below. This destroys the medial branch of the dorsal ramus and its accompanying vascular supply at the level of fusion, as well as one level above and below. This is yet another way to destroy muscle function because a muscle deprived of its neural and vascular supply tends to atrophy. Over the years that follow surgery, this degeneration of the multifidus, transfers its normal multi level functions to other surrounding muscles not designed for the role normally performed by the multifidus, overworking these other muscles and becoming probably a new source of pain and dysfunction. All spinal surgery and constructs that use the midline posterior surgical approach and its consequent resection of the multifidus, share this problem. In creating morbidity at the segment to be fused, it compromises function at levels above and below, which can effect the entire spine and the patient's wellbeing over the years to come.
In order to avoid the posterior midline approach's damage to the posterior muscles, ligaments, vessels and nerves, some surgeons approach the spine from the abdomen and build a fusion construct placing inter-vertebral body cages from the anterior approach. This technique is called the Anterior Lumbar Interbody Fusion or ALIF and gives the surgeon the ability to achieve segmental lordosis which corrects spinal balance. The ALIF cages provide optimal anterior spinal column support, and places copious bone graft against the cleaned vertebral bony endplates of the vertebral bodies to be fused. The ALIF, when used alone without pedicle fixation, does not disrupt the posterior back muscles, nor does cage placement require decompression or trauma of the posterior structures of the spine.
However, the anterior approach has limitations and dangers of its own. Spinal canal stenosis, requiring significant boney decompression within the spinal canal for treatment, cannot be achieved from the anterior approach. Isthmic sponylolysthesis, where a vertebra slips anteriorly in relation to the one below because of a defect in the pars interarticularis prevent the facets joints from performing their normal breaking function, requires stabilizing in addition to the cages in order to compensate for the deficient facet joints. For ALIF patients, pedicle fixation that replaces the role of facets must be added from a separate posterior approach that disrupts the muscles. Therefore many cases of isthmic sponylolysthesis cannot be treated by the anterior approach, unless stabilization is added such as posterior pedicle fixation. This from the posterior midline, eliminates the ALIF's advantage of saving the muscles. From the ALIF approach, it is possible to disrupt certain neural structures, notably the Sympathetic Trunk which in male patients control function of the sexual organs and can cause retrograde ejaculation, a known complication of the anterior approach. Furthermore, within the community of surgeons who treat spinal disorders, not all are trained to perform abdominal surgery alone or to manage the life threatening blood loss due to injury of the major vessels located in the path to the anterior spinal column. Thus, many spine surgeons prefer to place their inter-vertebral-body cages from the posterior approach, regardless of the pathology, most often in a surgical procedure called Posterior Lumbar Interbody Fusion or PLIF, which in recent years, is most often associated with pedicle fixation.
Over the last twenty years, artificial discs have been developed in the hopes to restore motion and therefore function to a diseased spinal motion segment. Also referred to as spinal arthroplasty, the goal of these implants is to create an artificial joint. Here lies the limitation of current technology. A spinal motion segment consists of a three joint complex, consisting of two facet joints and the disc. In their present form, only the disc is replaced and most diseased motion segments have degeneration in all three joints as advanced disc disease often presents advanced facet hypertrophy and stenosis. The painless function of the artificial disc requires healthy function of the facets. Artificial facets could be considered, but a means to replace them would have to be found that does not destroy surrounding muscles required for the motion segment.
Surgical Techniques: PLIF
A PLIF is performed from a posterior midline incision. In the PLIF, prior art cages are placed bilaterally, in a straight, anterior to posterior direction which is parallel to the sagittal plane, passing through the posterior muscles, past the spine's partially resected posterior elements, entering the spinal canal, past the dura containing its nerves and vessels, to enter the space between two vertebral bodies, called the interspace. To insert bilateral PLIF cages, the surgeon must therefore dissect the multifidus muscles from the lamina, the spinous process, with the undesirable consequences described above. The surgeon must then enter the spinal canal by resecting the ligamentum flavum, part or all of the lamina, part or all of the lateral facet joints, which are all themselves stabilizing elements, protecting the dural sac containing spinal nerves within the spinal canal. Then the dura must be retracted and the venous plexus that surrounds is cauterized to prevent excessive bleeding of structures directly connected to the central vascular system. The stabilizing posterior longitudinal ligament and part of the annulus fibrosis is partially or completely removed. Only after significant trauma to the muscles, after surgeon induced instability which in themselves may be of no therapeutic value, and after a significant loss of blood, can the surgeon begin removing the cartilaginous interbody disc to prepare the bed for the bilateral intervertebral PLIF cages. The cages are then inserted in straight posterior to anterior direction, parallel to the sagittal plane, from each side of the now exposed spinal chord. Most often, the PLIF cages are combined with pedicle fixation to compensate instability created by the partial or complete removal of the facet joints and the dissection of the longitudinal posterior ligament.
It is usually thought that the magnitude of the patient's problem justifies the amount of soft tissue destruction. In certain cases the surgeon desires to remove these structures in order to decompress a specific nerve for treatment. Still a bilateral PLIF procedure obliges the surgeon to perform dissections that in some situations may have no therapeutic value other than access for intervertebral cages. Posterior structures can be destroyed simply for posterior access to the anterior spine.
On the other hand, a PLIF spinal fusion construct in conjunction with pedicle fixation, results in a highly stable structure consisting of two bilateral supports between the vertebral bodies, with a bone inducing substance at an optimal location and pedicle fixation that serve as substitutes for destabilized facets. Surgeons have searched for the same therapeutic gesture, a similar biomechanical construct, but with less extensive trauma to the surrounding anatomy which need not to be touched for treatment. Because in some situations the midline approach PLIF dissects structures that would be good to leave intact, refined midline decompression and sagittal cage insertion methods have been created which are also used with pedicle fixation.
Surgical Technique: TLIF
Some patients present symptomatic pathology which is located predominantly on one side, requiring a destabilizing nerve decompression for treatment within the spinal canal, but only on one side. At the same time, the surgeon must build for fusion a stable biomechanical construct which is bilateral. The goal in such cases is to assure the treatment of the lesion, disrupt only the structures required for treatment on one side, but stabilize both. The unilateral Transforaminal Lumbar Interbody Fusion (called TLIF by some) was developed to treat such pathology. For this technique, the surgeon resects one entire facet joint on only one side and then inserts the cages through the space created. The procedure is called Transforaminal because the posterior aspect of the intervertebral foramen is formed by the facet joint. Once removed, the cage passes here. For the TLIF technique, cages therefore are inserted from one side, anterior or posterior, again in a direction parallel to the sagittal plane, but this time translated to the midline from the side of insertion.
It is important to note that although the TLIF inserts the cage from one side, where the facet has been resected, this procedure does not use the Extraforaminal Lumbar Interbody Fusion (ELIF) Surgical Plane, described below for the surgical approach of this invention. Thus, to gain access to the facet to be removed, the unilateral spinal canal and then the more anterior disc space, the multifidus must be dissected from the posterior elements, from at least one side, which is the same dissection for each side of a PLIF. One main difference between the TLIF as compared to the PLIF, is that only one side of the spinal canal is entered for decompression, disc access and then cage placement. This results in less blood loss and less potential trauma to the nerves.
A second main difference from the PLIF is that the TLIF usually resects one entire facet from the side which surgeon wishes to enter the canal. This gives access to the disc but is much more destabilizing than the PLIF which most often tries to conserve the lateral portion of each facet in an attempted compromise between neural decompression and biomechanical stability. The dura is then retracted to the midline and the venous plexus cauterized from one side. Next, the posterior longitudinal ligament is cut from its most lateral aspect to the midline. Again, this is a partially destabilizing maneuver, which should be justified only in terms of its therapeutic or biomechanical value and not simply for access to the spine. The posterior annulus is after cut from the ring apophysis at the midline and laterally to where the resected facet was located. The disc is then cleaned of its cartilaginous material and a first cage is placed via the transforaminal approach. With special instruments the first cage is laterally translated to the midline. A second cage is inserted, which, using the same instruments, pushes and translates the first, further to the lateral side.
In this manner, a bilateral cage construct is achieved from a midline posterior approach by using only one side entry through the spinal canal. Because a one stabilizing facet joint has been removed for cage entry and decompression, pedicle fixation must be added at this side to perform the breaking function of what has been removed. At times, unstable spinal conditions will oblige the unilateral TLIF surgeon, to insert his cages from one side of the spinal canal, but to place pedicle fixation in both sides. In these instances, the spinal canal is only approached and disrupted on one side, while the multifidus is disrupted bilaterally. Thus in the cases where bilateral pedicle fixation is performed with a TLIF, the goal for less trauma is achieved for only boney structures, ligament and neural structures, and not the posterior lumbar muscles.
Prior art methods using the posterior midline approach are usually most logically reserved for those cases where the pathology requires the surgeon to dissect the posterior elements, such as a portion of facet, to access the spinal canal where decompression must be achieved for the purposes of treatment and not only for stabilization with cages.
It should be noted that final placement in these prior art methods insert the cages in a straight line that is parallel to the sagittal plane. The medical term for “sagittal” is the plane anterior to the posterior. This describes the orientation of most bilateral cages inserted from a midline surgical approach, either using bilateral placement that is from both sides of the dura and the spinal canal for the PLIF, or unilateral lateral placement which results in a bilateral construct where cages are placed from one side of the spinal canal, and translated to the contra-lateral side as in the TLIF.
The extensive dissection of the posterior midline approach, with its destruction to the surrounding muscles, ligaments, vessels and nerves, has lead surgeons to explore other surgical techniques. As a means to place pedicle screws, percutaneous pedicle screw insertion methods have been developed using trocars. As the location of the pedicle cannot be directly visualized, a Kirshner wire is driven through the skin, using intra-operative x-ray fluoroscopy for each pedicle. Dilating trocars are placed over the Kirshner wire until a sufficient opening has been achieved for the pedicle screw to run through the trocar. A small but separate incision is made for each pedicle. Pedicle screws are inserted through the trocar into the pedicle of each vertebrae to be fixed, on each side of the patient. A special jig is attached, and through yet another stab incision a connecting rod is threaded through each screw connector, all of which is blocked into one joining construct.
While this technique has the merit of allowing the surgeon to build a pedicle fixation portion of his construct without dissecting the multifidus from the posterior elements, it is achieved at the price of six separate incisions for a construct of four pedicle screws. It indiscriminately splits muscles rather than separating muscle groups along the natural line cleavage created by the muscle fascia, protecting the muscles vascular and neural sources. With this technique, access to the spine and its pathology is indeed minimal, that is insufficient to access and treat many lesions. To reach the interspace for cage placement, separate incisions are required. To place bone graft at the facet joints, about the pedicle screw, separate incisions must be made. In addition, because the entry to the pedicles cannot be directly visualized, a great deal of radiation must be endured by both patient and surgeon, as each screw placement must most often be validated by x-ray fluoroscopy. New virtual technologies, which allow the surgeon to “see” screw placement on a video screen, are being explored as an alternative to radiation guidance for each percutaneous screw. But again these technologies require yet another incision to place special markers on the anatomical landmarks of the spine.
The above described limitations of the prior art caused the inventors to consider surgical approaches and technologies which instead of attempting to constrain the anatomy to an implant, used the body's natural delineations to achieve maximum access to the posterior spine, yet with minimal trauma. A stable fusion construct that reduces trauma allows the patient to move sooner, while protecting the fragile healing bone to improve the chances of the fusion growth and assure a life living bridge of bone. The surgical approach to make it possible should, wherever possible, limit dissection to the treatment of the pathology, without compromising the stability of the construct and the extent that the compressive pathology is removed. The resulting construct must respect the overall balance of the spine.